This invention relates to the support bodies for layers of individual conductors of a low temperature cable in general, and more particularly to an improved support body construction which provides better cooling and better support.
Support bodies for use with low temperature cables, such as super conductors in wire or ribbon form which consist of a plurality of ring shaped carrier elements arranged one behind the other in the axial direction with connecting parts arranged therebetween to form a common tubular molding having an outside diameter smaller at the connecting points than at the carrier elements are known. In both d-c and three phase cables in which the conductors are to be cooled to a low temperature, e.g., super conductors, it is advantageous to make the cables flexible to permit winding on the cable drums and facilitate laying in the field. In order to obtain flexibility, it is common practice to use one or more layers of individual conductor wires or ribbons. For example, individual aluminum wires which are covered with a super-conducting layer on their outer surface can be used. The layers of conductors or wires are advantageously arranged on the outer circumference of support bodies. Furthermore, they may be twisted such that a length of the cable which is independent of temperature during cooling of the cable to a very low temperature is possible. Such is disclosed, for example, in U.S. Pat. No. 3,541,221.
In U.S. Pat. No. 3,668,297, a flexible low temperature cable is disclosed in the form of a tubular conductor comprising individual coil conductors. The individual conductors have cross sections which are approximately rectangular and are arranged with mutual spacing side by side on the outer circumference of a support body of plastic. Illustrated support bodies include support rings which may be held together using wires, or a support helix comprising a wide plastic helix on which a further narrow helix may also be placed.
Another support arrangement is disclosed in U.S. Pat. No. 3,864,508. Moldings of annular cross section with longitudinally arranged slots at their outer surfaces are used as support elements for the longitudinally arranged helically wound conductor strands. The individual moldings are joined together through the use of connecting links to form a link chain. Between the individual moldings, provisions are made for the passage of a cooling medium. Clamps or wires, for example, may be used as connecting links.
Another support arrangement is disclosed in U.S. Pat. No. 3,691,287. This arrangement includes a plurality of support rings which may be combined with a wrapping of foils or tapes to form a supporting, hose-like structure. In addition, these support rings may be connected with each other loosely through the use of rods.
To manufacture a low temperature cable using these known support bodies however, rigid guidance means are required if the ribbon shaped or wire shaped individual conductors are to be placed on the outer surface of the support bodies. Since the necessary guide means must be provided in the available cabling machines over long distances of, for example, 20 meters, difficulties arise in wrapping the support bodies. Specifically, elongations or buckling of the helical support body and axial displacement of the support rings or annular moldings must be prevented by taking additional special measures at the cabling machine. Furthermore, the known types of support bodies must be mechanically stable in order for continuous production of a cable to be possible.
If, in addition, the individual support rings or annular support elements are held together using metallic connecting links such as clamps, and if the super-conductors wound on the support body are operated with a-c current, additional eddy current losses can be produced by the residual magnetic field in the support body.
There is a further danger in a cable with such support bodies that the individual support rings or annular moldings may shift in the event of temperature gradients, such as those occurring during cooling or warming processes. The larger spacing which then occurs between adjacent support rings or annular moldings along with diameter changes occurring in the support body of helical design result in an inferior placement of the individual conductors. As a result, a danger exists that the conductors can move and thereby that individual conductor sections can become normally conducting. With a type of support body taught in U.S. Pat. No. 3,730,966, displacements between individual support rings or annular moldings cannot occur. In the disclosed arrangement a corrugated central structure of arbitrary length is used as a support body. This structure consists substantially of a hollow, cylindrical plastic body which has radial constrictions at regular spacings in the longitudinal direction. Thus, ring shaped carrier elements are produced which are held together by means of connecting parts of somewhat smaller diameter which form a common molding therewith. The carrier elements in the connecting parts have approximately the same axial width. Furthermore, the connecting parts are each provided with rectangular openings, arranged one behind the other in a circumferential direction, to allow access for a cryogenic medium flowing inside the support body to the conductors placed on the support body. Contact with the medium occurs only in the radial direction in the vicinity of the constrictions of the connecting parts of the support body. However, an axial flow of the cryogenic medium in the longitudinal direction below the conductor layer which is placed on the support body is not possible since the conductor layer surrounds the carrier element with a tight fit. This results in an increase of the flow resistance of the cryogenic medium in the axial direction and cooling of the conductor layer becomes more difficult. If a conductor layer with superconductive material is placed on this known support body, then the heat occurring in the event of a quench, i.e., if the superconductive material becomes normally conducting, cannot be removed fast enough. Thus, in the cable taught in U.S. Pat. No. 3,730,966 direct placement of the superconducting individual conductors on the support body is not provided.
Thus, it can be seen that none of these prior art arrangements are completely satisfactory. In view of this, the need for an improved support body providing better cooling and better support becomes evident.